Eliminating patterns from and hardening of shell molds



1970 c. BENSON T AL 3,519,057

ELIMINATING PATTERNS FROM AND HARDENING OF SHELL MOLDS Filed Aug. 8, 19s? INVENTORS CHARLES L. BENSON WILLIAM M LENAHAN ATTOR N EYS United States Patent O 3,519,057 ELIMINATING PATTERNS FROM AND HARDENING OF SHELL MOLDS (Iharles L. Benson, Goshen, N.Y., and William. N. Lenahan, Denville, N.J., assignors to Howmet Corporation, New York, N.Y., a corporation of Delaware Filed Aug. 8, 1967, Ser. No. 659,182 Int. Cl. 1322c 9/04 US. Cl. 164-35 1 Claim ABSTRACT OF THE DISCLOSURE A method of making a refractory shell mold on a fusible pattern wherein the mold is soaked with a flammable liquid, the liquid is burned to heat the mold and melt out the pattern material, and then before the mold is permitted to cool it is fired to the desired hardness in a furnace.

BACKGROUND OF THE INVENTION Field of the invention The art to which this invention pertains is the manufacture of refractory shell molds. It involves making a pattern of the desired casting in wax or similar readily fusible material, then forming a refractory shell mold by dip-coating the pattern a number of times with a liquid suspension of a fine-grained refractory which hardens on the pattern, then eliminating the pattern from within the mold built up in this manner, and finally firing the mold to the desired hardness in a furnace. In particular, the pattern is melted out of the mold by burning a flammable liquid soaked into the mold, and the invention relates to improvements in that step of eliminating the pattern and the subsequent step of firing the mold in a furnace. The improvements are predicated on the concept of firing the mold in the furnace very promptly after the flammable liquid has burned, or even commencing the firing prior to completion of the burning of the flammable liquid, so that the mold is not permitted to cool between the steps of burning the flammable liquid and firing in the furnace. By avoiding such cooling, little or no carbonized residue is allowed to form as a result of burning the flammable material and the time required for firing is appreciably reduced since only a minimum of oxidized material need be eliminated in the furnace.

Description of the prior art It has been known to be advantageous in the manufacture of refractory shell molds to eliminate the fusible pattern material by soaking the mold with an inflammable liquid and then igniting the liquid. The resulting combustion heats the mold in a fashion which not only melts out the pattern material but does so with such rapidity that the pattern undergoes little thermal expansion and hence leaves the mold intact. Oftentimes this flammable liquid is gasoline, hexane, acetone, alcohol or other liquid fuel which is dipped, sprayed or otherwise suitably soaked into the mold. In other instances, such as where the mold is formed of a gelled siliciferous material, it is helpful to provide a flammable liquid which leaves a silica residue having the function of strengthening the mold somewhat in its green state prior to firing in a furnace; for example, a mold formed by successive dipping of the pattern in a slurry having a binder of hydrolyzed ethylsilicate or isopropyl silicate can be strengthened in its green state if it is heated, by burning a siliciferous liquid such as an acetone or ethanol based hydrolyzed ethyl silicate. British patent specification No. 800,143 describes a method of eliminating patterns from shell molds which embodies these principles.

As the flammable liquid burns, some of the melted wax pattern material soaks through the mold and is left in a partially oxidized state when the mold cools. An undesirable result of this is that the residue of Wax left on the mold in this manner forms into very hard carbonized matter during the subsequent firing step and requires a considerable period of time to be burned away. Heretofore it was believed that a protracted firing time of several hours was necessary to achieve complete oxidation of this carbonized material.

SUMMARY OF THE INVENTION The present invention is directed to an improvement in methods of making a refractory shell mold as described above wherein the mold is formed on a fusible pattern and then soaked with a flammable liquid, the liquid is burned toheat the mold and melt the pattern material so that it flows therefrom, and the mold is fired to the desired hardness in a furnace. The improvement comprises firing the mold in the furnace without first permitting substantial cooling thereof subsequent to the burning of the flammable liquid. Preferably, the mold is located in the furnace during burning of the liquid and the firing is commenced before the burning is completed. By avoiding any interruption which allows the mold to cool after the flammable liquid has been burned but before the mold is fired, very little partially oxidized wax is left on or soaked in the mold to form hard carbonized material. The overall process is therefore greatly shortened, not only because cooling time is avoided but because lesser firing time is required for burning away pattern material. It has been discovered that a substantial amount of thecarbonized material formed on the mold is a direct result of the interim cooling stage, and it is avoided in accordance with the invention by insuring that the mold is not allowed to cool before it is fired.

DESCRIPTION OF THE DRAWING The following description of a preferred embodiment of the invention has reference to the accompanying drawing which is a schematic view of the operational stages of a mold forming process as contemplated by the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A pattern of the desired casting is made of wax, a thermoplastic composition, or other suitable material. The pattern is clipped in to a slurry comprising a silicacontaining binder and some other refractory (not necessarily silica) and the dip-coating adhering to the pattern is dusted with a relatively coarse refractory. The slurry dip-coat may have a binder of hydrolyzed ethyl silicate or isopropyl silicate and be gelled in an atmosphere of ammonia gas for two or three minutes, or it may be bonded with colloidal silica. Successive dip-coatings are applied until a shell mold is built up on the pattern. At this point the mold is in the green state and the fusible pattern is to be eliminated.

The method of eliminating the pattern and firing the mold in accordance with the invention can be readily carried out on a conveyor 10 as shown in the accompanying drawing. It is adapted to carry molds 11 through a series of process stages indicated as 11A through 11G in the drawing. In the first stage at 11A the mold containing a fusible pattern is soaked by dipping, spraying or other suitable technique with a flammable liquid. The liquid may be a fuel such as gasoline, hexane, acetone or alcohol, any of which is appropriate for use when colloidal silica is used as a bond for the refractory ofthe shell mold. Alternatively, when the slurry contains hydrolyzed ethyl or isopropyl silicate as a binder, the mold can be significantly strengthened by the use of a siliciferous flammable liquid such as an acetone or ethanol based hydrolyzed ethyl silicate, an isopropanol based colloidal silica, or an ethanol based hydrolyzed isopropyl silicate. This soaking of the mold with a flammable liquid is shown for purposes of illustration in the drawing as taking place on the conveyor, but it may be done elsewhere.

After the soaking step is carried out, the mold containing the pattern permeated with flammable liquid is placed on the conveyor 10 immediately ahead of an extended high temperature furnace 12 (e.g. about 1500 F.) through which the conveyor travels. In the position shown at 11B in the drawing the flammable liquid with which it is soaked is ignited, perhaps beneath a suitable hood 13 or other means for carrying off the gaseous products of combustion. Almost immediately, the conveyor 10 carries the mold into the furnace 12, where, as shown at 11C, burning of the flammable liquid continues. The heat given off by the burning liquid is transferred so rapidly to the pattern that before the bulk of the pattern can undergo critical thermal expansion its outer surface liquefies and drains off. A suitable pan 14 is provided beneath the conveyor 10 in this area at the beginning of the furnace 12 to collect the pattern material melted from the molds and drain it off to a suitable vessel 15.

Combustion of the flammable liquid may continue as the mold is carried by the conveyor 10 to a position shown at 11D, but by the time the mold advances appreciably into the furnace 12 as shown at 11E all of the flammable liquid has been consumed and the last remaining flowable pattern material has been melted out and carried away. By this time the heat of the furnace has taken effect and the refractory material begins to undergo hardening. No part of the mold is allowed to cool to any significant degree after combustion of the flamma ble liquid as shown at 11D and before heating by the furnace atmosphere. Consequently, virtually no carbonized Wax residue forms anywhere on or within the mold. It has been discovered that such formations will not take place when interim cooling is avoided.

The mold then continues on through the furnace 12 until its full high temperature strength has been developed near the exit of the furnace as shown at 11F. By that time, whatever remaining pattern material may have been left clinging to the mold surface has been oxidized and completely removed without going through a difficult stage of burning away dense carbonized matter. At the conclusion of the process, the mold emerges at 11G completely hardened, free of all undesirable residue, and emptied of its pattern. The total time required for the process from the soaking step at 11A to completion at 11G is far less than for prior methods where the mold was cooled before firing, even disregarding the hold time taken for cooling in the prior method.

What is claimed is:

1. In a method of making a refractory shell mold wherein the mold is formed on a fusible pattern and then soaked with a flammable liquid, the liquid is burned to heat the mold and melt the pattern material so that it flows therefrom, and the mold is fired to the desired hardness in a furnace, the improvement which comprises (a) firing the mold in the furnace without first permitting substantial cooling thereof subsequent to the burning of the flammable liquid, the mold being located in the furnace during at least the latter part of the burning of the liquid, and the firing being commenced before the burning is completed whereby the formation of any substantial amount of carbonized matter is prevented.

References Cited UNITED STATES PATENTS 2,930,089 3/1960 Emblem 16416 3,348,605 10/1967 Heron 164-35 FOREIGN PATENTS 716,394 10/1954 Great Britain.

1. SPENCER OVERHOLSER, Primary Examiner J. S. BROWN, Assistant Examiner 

